Hey, I don't know if you've gotten any help with this yet. I was having the same problem, even after disabling all the auto shutoff options. What finally fixed it is I turned of "auto protection." I have no idea what that is. It's in the settings.
To clarify / correct, it's "Auto Protection Time." This is a feature to turn off the tv when it detects a still image, to prevent screen burnout. Why does a device intended to display still images have this feature, and default it to "on"? I don't know. I believe the manual specifically says that screen burnout is not a concern with this tv. But you can check that.
Just got a 50" 2020 model and same problem. I resolved this problem when I turned off the Night Mode setting in the Art Mode settings. Seems overly sensitive because I do have lights on. They're dimmed slightly, but it is in no way dark in the room. Anyway, that fixed it for me!